49 
being, their structure, their physiology, and their habits ; 
affect them also at different periods of their life in a different 
way, and in regard to different elements in their constitution. 
Every species being thus subject throughout its whole life to 
an immense variety of stringent tests by which its relative pre- 
dominance is determined, the result of this must plainly be, on 
the average, the maintaining each species at its highest pitch 
of perfection in respect to the particular conditions to which 
it is exposed. With this, however, will always be also a 
distinct tendency to preserve variations, even of the slightest 
kind. In any given area the largest number of individuals, 
whether of plants or animals, will be found capable of co- 
existing, when the differences between them are at their great- 
est. It is well known, for instance, that a heavier crop of hay 
is obtained from a field sown with mixed seed than from one 
sown with only a single kind, simply because in the former 
instance more individual plants are capable of growing together 
than in the latter. The tendency of the struggle for existence 
being, of course, to preserve in every case the largest number 
of individuals possible, there will thus be an intrinsic advan- 
tage in every variation, apart from any positive bearing it may 
have on the well-being of the species. The severity of the 
struggle with individuals of the same species will at all events 
be diminished, and so a greater chance of preservation be 
afforded. Thus, even supposing no change to take place in 
the conditions of life, it is quite credible that natural selection 
should so seize hold of and confirm even indifferent variations 
as to make them permanent. How much more if they are of 
a kind directly profitable. 
But the conditions of life do not remain unchanged. The 
development of a new variety or species in the manner just 
noticed, the diminution or extinction of another by deteriora- 
tion (for species certainly vary in both directions), the immi- 
gration or chance introduction of some foreigner previously 
unknown there, would at once alter the relations of each 
species to the other, and so affect the kind of test by which 
their predominance was determined. Irregularity in the 
seasons might give especial advantage to some individuals 
and races, especial disadvantages to others, and thus tend to 
extinguish certain variations and preserve others, besides 
leading to internal alterations of relation. Changes in physical 
geography brought about by geological forces would be still 
more potent, as producing differences in the conditions of life 
more permanent and extensive. A greater or less elevation, 
an altered flow of rivers, a different course . of ocean currents, 
the connection or disconnection of land with land, — all w r ould 
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